Non-impact printers used in information processing equipment are well known at present. These machines include a recording carrier, most often comprising a rotary drum or an endless belt, on the surface of which sensitized zones, also known as latent images, can be formed electrostatically or magnetically, corresponding to the characters or images to be printed. These latent images are then developed, or in other words made visible, with the aid of a powdered developer, which when deposited on the recording carrier is attracted only by the sensitized zones on it, thus forming an image in powder on the surface of the carrier. Next, the recording carrier is put in contact with a sheet of paper, so that the developer particles comprising the powdered image can be transferred to the sheets and definitively fixed there.
The application of developer particles to the recording carrier in printing machines of this type is accomplished by applicator devices of a known type, such as that described in U.S. Pat. No. 4,246,588 (corresponding to French Patent No. 2.408.462). These applicators generally include a transport means that as it travels past an opening in the upper portion of the reservoir containing the particles picks up particles located in the reservoir and moves them to the vicinity of the surface of the recording carrier. With these applicators, however, despite all the care taken in their construction, it is difficult to prevent the developer particles from being deposited not only to excess over the sensitized zones of the recording carrier but also, although in very small amounts, outside these zones. For this reason the printers are also provided with a retouching device, which is disposed between the particle applicator and the station where the particles are transferred to a sheet of paper and makes it possible to remove the excess developer particles located on the surface of the recording carrier. Although retouching devices capable of performing the retouching by magnetic attraction or by blowing air have been made, the preference at present is for retouching devices that function by air suction and have the advantage of being non-polluting and of enabling removal of the excess particles on the recording carrier surface, without requiring that the particles have magnetic properties in order to accomplish this. Furthermore, some of these retouching devices are associated with transport devices that enable the developer particles, thus removed, to return to the particle reservoir for recycling. Hence a retouching device has been embodied as described in U.S. Pat. No. 4,046,682, which includes both a suction conduit provided with a slit or nozzle, extending in proximity with the surface of the recording carrier, and an opening connected via a duct to a suction turbine. This device also includes an endless belt, made of a material permeable to air, which is driven continuously and passes through the duct. Under these conditions, when the turbine rotates, the excess developer particles on the portions of the recording carrier located vertically of the slit in the suction conduit are transported by the air aspirated by the turbine and as they circulate in the duct are stopped in their passage by the endless belt. The particles are virtually all trapped in the meshes of the material comprising the belt and are then carried by it. In the course of its path, this belt travels under the reservoir holding the particles; when pressed against the edge of a scraper disposed in the upper portion of the reservoir the belt causes the particles with which it is impregnated to drop into the reservoir. However, this procedure, which enables developer particles from the retouching operation to be reintroduced into the reservoir, does not give complete satisfaction; not only is it impractical and always incomplete to recover the particles trapped by the belt, but the portion of the conduit located between the belt and the suction nozzle becomes more or less plugged eventually, which notably lessens the output of the suction turbine. Furthermore, the belt wears very rapidly and must be frequently replaced, with difficulty, which means a higher maintenance cost for the machine in which the belt is used.
The present applicant has recently succeeded in overcoming the problem of separation and recovery of particles by replacing the endless belt with a negative-pressure separator apparatus of a known type, typically known as a cyclone, this apparatus (an improved version of which is the subject of a patent application filed on the same date as the present one by the present applicant) includes a separation chamber provided in its lower portion with a discharge conduit, which is normally closed on its lower end by a movable valve in order to comprise a box in which the particles that have been removed from the gaseous carrier flow accumulate. However, it has been observed that when this apparatus is disposed above the reservoir of particles, each time the valve is opened to allow the recovered particles to drop into the reservoir, a rush of air ensues, because on the one hand the reservoir is necessarily provided with an opening that, since it is intended for the passage of the particle applicator device has the effect of connecting this reservoir with the open air, and on the other hand the separation chamber is at negative pressure with respect to the ambient atmospheric pressure, the value of this negative pressure being on the order of 200 hectopascals. This rush of air not only interferes with the functioning of the cyclone but also entrains particles contained in the reservoir, so that they rise into the separation chamber again. These particles are then aspirated by the air that leaves the separation chamber normally, and being thus entrained they pass through the turbine and are then ejected into the atmosphere, which is not only polluting but in the long term fouls the turbine.